A fuel cell 1 of this type of direct methanol fuel cell (DMFC) is configured as shown in FIG. 4.
The fuel cell 1 of the fuel cell is formed with electrode catalyst layers 3, 4 on both surfaces of a solid polymer electrolyte 2, and is configured so as to supply methanol aqueous solution 5 to one surface of the solid polymer electrolyte 2 via the electrode catalyst layer 3 on one side of the solid polymer electrolyte 2 and to supply oxygen 6 to the electrode catalyst layer 4 on the other side of the solid polymer electrolyte 2 (Kogyo Chosakai Publishing Inc., “ELECTRONIC PARTS AND MATERIALS”, February 2003, p. 31, “PORTABLE COMPACT FUEL CELL”, Nobuyuki Kamiya).
The principle of electric power generation will now be explained.
The fuel cell 1 is formed with a fuel electrode (anode) 7 including the electrode catalyst layer 3 on one side of the solid polymer electrolyte 2, and an air electrode (cathode) 8 including the electrode catalyst layer 4 on the other side of the solid polymer electrolyte 2. When methanol aqueous solution of the fuel is supplied to the fuel electrode 7, a chemical reaction proceeds at the fuel electrode 7, so that carbon dioxide, protons and electrons are produced.CH3OH+H2O→CO2+6H+6e−
After the protons transmit through the solid polymer electrolyte 2, and the electrons pass through the external circuit, a chemical reaction proceeds at the air electrode 8 with the protons, the electrons, and the oxygen supplied to the air electrode 8, thereby producing water.
The stoichiometric composition of methanol of the methanol aqueous solution and water supplied to the fuel electrode 7 is 1:1 in molar ratio (64% by weight of methanol aqueous solution), but since cell output lowers as the methanol transmits through the solid polymer electrolyte 2 and reaches the air electrode 8 (methanol cross over), the actual optimum concentration is about 3 to 30% by weight, and thus water tends to remain when methanol and water react for electric power generation.
Supply of methanol aqueous solution fuel includes a “liquid fuel natural flow system (passive DMFC system)” in which a liquid chamber of the fuel electrode 7 is filled with methanol aqueous solution, and the fuel is directly supplied to the liquid chamber of the fuel electrode 7 from a tank in accordance with the consumed amount, or a “liquid fuel circulatory system (active DMFC system)” in which the methanol aqueous solution fuel is passed through and circulated in the liquid chamber of the fuel electrode 7, and the methanol fuel is supplied from the methanol fuel tank, in accordance with the consumed amount, to the circulatory tank communicating with the liquid chamber (refer to Japanese Patent No. 2939978, JP-A 10-507572).
Comparing the two systems, the passive system is suited for compactness since a section for circulating the fuel is unnecessary, but as the methanol aqueous solution reacts at the electrode catalyst layer 3 of the fuel electrode 7, methanol is consumed, thereby lowering the methanol concentration and the fuel output. The active system has the liquid fuel circulate in the liquid chamber of the fuel electrode 7, and thus has an advantage of obtaining an output concentration of about 3 to 5 times the passive system.
FIG. 5 cites (Patent Document 2), where the high-concentrated methanol is set in a methanol reservoir tank 9, which methanol reservoir tank 9 is connected to a circulatory tank 11 via a fuel+water injecting device 10. A circulating path is formed so that the methanol aqueous solution 5 taken out from the circulatory tank 11 with a pump 12 returns back to the circulatory tank 11 via the fuel electrode 7 of the fuel cell 1 and a heat exchanger 13. Further, air is supplied to the air electrode 8 of the fuel cell 1 via an oxidant supplying device 14.
Therefore, at the fuel cell 1, the methanol aqueous solution causes the cell reaction, thereby consuming methanol and producing oxygen dioxide. The mixed fluid of low-concentration methanol solution and gas is returned back to the circulatory tank 11, where the liquid and the oxygen dioxide gas are gravity separated, and the oxygen dioxide gas is then released from the upper part of the circulatory tank 11. On the other hand, since the methanol and water are consumed by the cell reaction thereby reducing fuel, a predetermined amount is injected from the fuel+water injecting device 10 into the circulatory tank 11 at a concentration of optimum methanol fuel.
The water produced by the reaction caused by the air supplied via the oxidant supplying device 14 to the air electrode 8 of the fuel cell 1 is collected at a water collecting device 15 and returned back to the fuel+water injecting device 10, and again used as the water to be supplied to the fuel electrode 7. An air pump is usually used as the oxidant supplying device 14.
In such active system, a liquid pump is used as the pump 12 for passing and circulating the methanol aqueous solution fuel to the liquid chamber of the fuel electrode 7, and when air or bubbles are mixed in the flow path, troubles such as, malfunctioning and stopping occur, or failure by foreign materials occur. The dissolved gas of the liquid in the flow path may be generated as bubbles while the pump 12 is stopping, thereby causing malfunction. The liquid pump has higher drive torque compared to the air pump, and thus has a short bearing life span and the performance lowers in a continuous operation of a few hundred hours. Further, when the orientation of the fuel cell is turned upside down, air contacts the outlet of the methanol aqueous solution of the circulatory tank 11, or methanol aqueous solution contacts the outlet of the oxygen dioxide, thereby inhibiting the discharge of the methanol aqueous solution.
Further, the separation and release of the oxygen dioxide gas generated when the methanol aqueous solution causes cell reaction at the electrode catalyst layer 3 of the fuel electrode 7 of the fuel cell 1 are not easily performed, and becomes the cause of lowering of cell output.
When the liquid pump is used to refill the methanol of high concentration of the methanol reservoir tank 9 to the circulatory tank 11, transportation of a small amount of liquid is difficult, and troubles such as, bubbles may arise thereby inhibiting transportation of methanol of high concentration, and further, when the orientation of the fuel cell is turned upside down, air may contact the methanol outlet of the methanol reservoir tank 9 thereby inhibiting the methanol from being discharged. The methanol of high concentration is expensive since a pump made from a special material having resistance to methanol must be used.
The liquid pump is also used in injecting water collected at the water collecting device 15 to the fuel+water injecting device 10, and troubles such as bubbles may arise thereby inhibiting the water from being transported, or when the orientation of the fuel cell is turned upside down, the air may contact the water outlet thereby inhibiting the water from being discharged.
The present invention aims to provide a method of generating electric power of an active fuel cell in which troubles caused by mixing of air or bubbles in the flow path are alleviated, and in which a stable operation can be expected over a long period of time.
The present invention also aims to provide a method of generating electric power for reliably performing the separation and release of the oxygen dioxide gas generated when cell reaction occurs at the electrode catalyst layer of the fuel electrode 7 of the fuel cell 1.
The present invention also aims to provide a method of generating electric power capable of transporting a small amount of methanol of high concentration of the methanol reservoir tank to the circulatory tank without troubles of bubbles and the like as seen with the liquid pump, and that can reliably refill the methanol of high concentration to the fuel electrode irrespective of the orientation of the fuel cell.
The present invention also aims to provide a method of generating electric power that has no troubles of bubbles and the like as seen in the liquid pump, and that reliably injects water irrespective of the orientation of the fuel cell when injecting water collected in the water collecting device 15 to the fuel electrode side.